1. Field of the Invention
This invention relates to a transistor and more particularly to a high-frequency bipolar transistor, and to a method of fabricating the transistor.
2. Description of the Prior Art
A trend of semiconductor device technology is toward high speed and high frequency, as well as high integration. As is well-known, the high-frequency limitation of a bipolar transistor is usually described by the maximum oscillation frequency f.sub.max, which is expressed as follows: EQU f.sub.max =[4.pi.(R.sub.b C.sub.bc t.sub.ec).sup.1/2 ].sup.-1( 1)
where R.sub.b is the base resistance, C.sub.bc is the base-collector junction capacitance and t.sub.ec is the emitter-to-collector delay time. Since t.sub.ec also contains the term proportional to C.sub.bc, f.sub.max is especially sensitive to C.sub.bc. Accordingly, the reduction of C.sub.bc is essential to improve the high-frequency performance of a bipolar transistor. In the case of an emitter-top transistor whose emitter is located on top of the transistor, C.sub.bc is expressed by the sum of an intrinsic capacitance C.sub.bci and a parasitic capacitance C.sub.bcp, as follows: EQU C.sub.bc =C.sub.bci +C.sub.bcp ( 2)
The portion of a base region which underlies an emitter region is called an intrinsic base region, the rest of the base region, which is extended from the intrinsic base region, is called an extrinsic base region. Therefore, the intrinsic base region is an active region, and the extrinsic base region is an inactive region. C.sub.bci is the junction capacitance formed between the intrinsic base region and the collector region. C.sub.bcp is the junction capacitance formed between the extrinsic base region and the collector region. Reduction of C.sub.bci is, however, quite difficult because C.sub.bci is almost automatically determined by the requirement to realize a fundamental transistor action. Accordingly, the possible choice is limited to reduce C.sub.bcp.
Recently, AlGaAs/GaAs heterojunction bipolar transistors (HBT's) have been attracting much interest because of its outstanding possibilities for high-frequency devices. In such HBT's, the bandgap of a material of the emitter region is wider than that of a material of the base region, so that the emitter-base junction forms a heterojunction. The principal advantage of the HBT's over Si homojunction bipolar transistors is that sufficiently high base-doping is possible without sacrificing the emitter injection efficiency, because the minority carrier injection from the base region to the emitter region is suppressed by the wider emitter bandgap of the heterojunction. Accordingly, both the base width and R.sub.b can simultaneously be reduced. As is well-known, the reduction of the base width is especially effective to decrease the base transit time, which is generally the most important component of t.sub.ec. As a consequence, f.sub.max can be increased according to the formula (1). The other advantage is that the electron mobility of GaAs is faster than that of Si.
One of the conventional methods to reduce C.sub.bcp in HBT's is, as shown in FIG. 1, to introduce a semi-insulative region 20 between an extrinsic base region 21 and a collector contact region 11. (Ref. P. M. Asbeck, D. L. Miller, R. J. Anderson and F. H. Eisen, IEEE Electron Device Letters, vol. EDL-5, p. 310, 1984.) Here the semi-insulative region means that its resistivity is sufficiently high compared with those of n-type or p-type transistor regions. In the prior art cited above, a collector contact region 11 made of n.sup.+ -GaAs, a collector region 12 made of n-GaAs, a base region 13 made of p.sup.+ -GaAs, an emitter region 14 made of n-AlGaAs and an emitter contact region 15 made of n.sup.+ -GaAs are fabricated in this order on a semi-insulating (s. i.) substrate 10. Be.sup.+ is implanted in the extrinsic base region 21 to reduce R.sub.b. A collector electrode 17, double base-electrodes 18 and an emitter electrode 19 are formed upon the collector contact region 11, the extrinic base region 21 and the emitter contact region 15, respectively. The conduction property of the portion 20 sandwiched between the extrinsic base region 21 and the collector contact region 11 is changed from n-type to semi-insulative by 0.sup.+ implantation. Accordingly, C.sub.bcp is reduced as the base-collector junction is thus changed from p.sup.+ -n to p.sup.+ -i-n.sup.+.
In the above structure, however, the reduction of C.sub.bcp is limited to a certain extent, because the overlap between the extrinsic base region 21 and the collector contact region 11, which makes the p.sup.+ -i-n.sup.+ junction, still remains.